Electronic device with heat conductive encasing device

ABSTRACT

The invention relates to the field of electronic devices with a thermally conducting encapsulant for draining away some of the energy dissipated by the electronic components contained in the electronic device. This is an electronic device comprising: a circuit ( 7 ) on which several electronic components ( 8 ) able to dissipate energy are placed; a thermal conducting cover ( 1 ) located opposite the circuit ( 7 ); a thermally conducting encapsulant ( 4 ) placed between the circuit ( 7 ) and the cover ( 1 ) so as to ensure heat transfer, by conduction toward the cover ( 1 ), of the energy dissipated in the components ( 8 ); the respective surfaces of the encapsulant ( 4 ) and of the cover ( 1 ) which are facing each other including a number of substantially complementary recesses ( 3, 6 ) and projections ( 2, 5 ) allowing the cover ( 1 ) to fit into the encapsulant ( 4 ), and gaps (j 1 , j 2 ) being left between the recesses ( 3, 6 ) and the projections ( 2, 5 ) so as, on the one hand, to reduce the stress exerted by the cover ( 1 ) on the encapsulant ( 4 ) in the direction of the circuit ( 7 ) and on the other hand, to maintain the thermal conduction between the encapsulant ( 4 ) and the cover ( 1 ) above a given conduction threshold.  
     The invention can be applied in particular to digital electronic cards.

[0001] The invention relates to the field of electronic devices with athermally conducting encapsulant for draining some of the energydissipated by the electronic components contained in the electronicdevice. The thermally conducting encapsulant generally drains away thedissipated energy toward a cover that is also thermally conducting. Thethermally conducting encapsulant also usually allows homogenization ofthe hot spots that are due to more localized energy dissipation incertain components.

[0002] According to a first prior art, the thermal encapsulant is aone-piece thermal block whose face on the component side is plane.However, when the size of the circuit of the electronic deviceincreases, the number of components on the circuit also increases andthe height of the various components becomes increasingly variable, thatis to say becomes increasingly different from one component to another.The thermal efficiency, that is to say the efficiency of drainage fromthe electronic device of the energy dissipated in the electroniccomponents, of such a one-piece thermal block with a plane face on acircuit the height of the components of which is relatively variable islow.

[0003] At the present time, electronic devices dissipating a great dealof energy generally operate with forced ventilation. However, it wouldbe of interest to be able to reduce or eliminate this forcedventilation. This is because forced ventilation requires a fan in oroutside the electronic device, consumes energy and makes the environmentof the electronic device noisy. However, reducing or eliminating theforced ventilation decreases the thermal efficiency, by reducing thedrainage of energy by convection. An electronic device with improvedthermal efficiency would be of interest if it allowed the forcedventilation to be reduced or eliminated.

[0004] According to a second prior art, to improve the thermalefficiency, buried sinks may be added on the circuit of the electronicdevice. However, it is advantageous to be able to improve the thermalefficiency without redesigning the circuit contained in the electronicdevice, especially in the case of an already existing and optimizedcircuit.

[0005] According to a third prior art, the thermally conducting coverincludes protuberances so as to match the variable height of thecomponents on the circuit. Thermally conducting encapsulants are placedbetween the cover with protuberances and the components. Each electronicdevice requires a dedicated thermally conducting cover. To tailor aspecific cover for each type of electronic device entails a high cost.

[0006] The invention provides an electronic device of high thermalefficiency in which good thermal conduction is ensured between thecomponents, in which the energy is dissipated, and the thermallyconducting cover, into which the dissipated energy is drained. Just byreducing the thickness of the air interfaces present in the dissipatedenergy drainage path, so as to maintain the thermal conduction above agiven threshold required by the particular electronic device inquestion. The invention prevents excessive mechanical stresses beingapplied to the electronic components, which are generally fragile, andalso being applied to other fragile elements such as, for example, thepoints where the connection leads of these components are soldered.

[0007] According to the invention, what is provided is an electronicdevice comprising: a circuit on which several electronic components ableto dissipate energy are placed; a thermal conducting cover locatedopposite the circuit; a thermally conducting encapsulant placed betweenthe circuit and the cover so as to ensure heat transfer, by conductiontoward the cover, of the energy dissipated in the components;characterized in that the respective surfaces of the encapsulant and ofthe cover which are facing each other include a number of substantiallycomplementary recesses and projections allowing the cover to fit intothe encapsulant, and in that gaps are left between the recesses and theprojections so as, on the one hand, to reduce the stress exerted by thecover on the encapsulant in the direction of the circuit and on theother hand, to maintain the thermal conduction between the encapsulantand the cover above a given conduction threshold.

[0008] The invention will be better understood and further features andadvantages will become apparent from the description below and from theappended drawings, given by way of examples, in which:

[0009]FIG. 1 shows schematically a preferred example of an electronicdevice according to the invention;

[0010]FIG. 2 shows schematically a preferred example of a thermallyconducting cover of an electronic device according to the invention;

[0011]FIG. 3 shows schematically a first preferred embodiment of adevice for at least partially covering the tooling circuit used in apreferred process for manufacturing a thermally conducting encapsulantof an electronic device according to the invention; and

[0012]FIG. 4 shows schematically a second preferred embodiment of adevice for at least partially covering the tooling circuit used in apreferred process for manufacturing a thermally conducting encapsulantof an electronic device according to the invention.

[0013] The electronic device comprises a circuit. Placed on this circuitare several electronic components. The electronic components dissipate,in operating mode, energy which must be drained away, at leastpartially, out of the electronic device. The components are relativelyfragile elements on which the exerted stress is reduced thanks to theinvention. The electronic device includes a cover. The cover is locatedopposite the circuit. The cover is thermally conducting, that is to sayit is sufficiently conducting to allow a substantial portion of theenergy dissipated in the components, and taken into the cover, to bedrained away to the outside of the electronic device. To take the energydissipated in the components into the thermally conducting cover, athermally conducting encapsulant is placed between the circuit and thethermally conducting cover. Thus, a substantial portion of the energydissipated in the components undergoes heat transfer by conduction tothe cover. The encapsulant is sufficiently thermally conducting to allowthe transfer of a substantial portion of the energy dissipated in thecomponents to the cover. A substantial portion of the energy is asufficiently large portion of the energy so that the thermal conductionbetween the components and the cover is maintained above a givenconduction threshold, which is determined by the type of electronicdevice envisioned, that is to say by the type of application envisioned.The circuit, the encapsulant and the cover form a stack whose layers areplaced substantially parallel to the mid-plane of the stack.

[0014] The cover has a surface facing the encapsulant and theencapsulant has a surface facing the cover. These respective surfaces ofthe encapsulant and of the cover which are facing each other include anumber of substantially complementary projections and recesses so as toallow interlocking of the cover and the encapsulant, one in the other.The projections of the cover fit into the recesses of the encapsulant,while the projections of the encapsulant fit into the recesses of thecover. If the recesses and the projections were to be completelycomplementary, there would be complete interlocking of the cover and theencapsulant and there would be no gaps between the recesses and theprojections, or the gaps that do exist would be negligible and of noappreciable effect. However, the size and the arrangement of therecesses and the projections are chosen so that the stress exerted bythe cover on the encapsulant in the direction of the circuit is reduced.This stress is reduced compared with the case in which there would becomplete interlocking. This stress is also reduced compared with thecase of the prior art since, in the prior art, the facing surfaces ofthe cover and the encapsulant are plane and bear on each other, exertingand transmitting a stress on the components that is at least as high as,if not higher than, in the case of complete interlocking of the recessesand projections with no gap or when there are negligible gapsinsufficient to substantially reduce the stress exerted on thecomponents.

[0015] The electronic device according to the invention makes itpossible to reduce or even eliminate the stress that is exerted on thecomponents and can damage them or interfere with their operation. In theprior art, this stress is caused by the encapsulant, generally quite ahard material having a Shore A hardness of, for example, typicallyseveral tons, pressing down on the surface of the circuit comprisingseveral or even many components whose height, that is to say the heightabove the level of the circuit, differs among the components or at leastamong some of them. During the mechanical handling of the electronicdevice, that is to say when it is being mounted, the stresses that mightbe exerted on the circuit by the cover via the encapsulant are reducedas they are at least partially absorbed by the gaps arranged between therecesses and the projections. These gaps are also arranged so that thereis not an excessive decrease in the thermal conduction between theencapsulant and the cover, or even so that this is increased in certainpreferred embodiments, and in any case so that the thermal conductionbetween the encapsulant and the cover is maintained above a givenconduction threshold required by the type of electronic deviceenvisioned, that is to say by the type of application envisioned. Thosegaps between the respective surfaces of the cover and the encapsulantthat are somewhat parallel to the mid-plane of the layers of the stackare large enough for the stress exerted by the cover on the encapsulantin the direction of the circuit to be reduced or even virtuallyeliminated. Those gaps between the respective surfaces of the cover andthe encapsulant which are somewhat orthogonal to the mid-plane of thelayers of the stack are small enough so that the thermal conductionbetween the encapsulant and the cover is maintained above the givenconduction threshold required by the type of electronic deviceenvisioned.

[0016]FIG. 1 shows schematically a preferred example of an electronicdevice according to the invention. The electronic device according tothe invention is composed of a stack of several layers arranged so as tobe parallel to the mid-plane of the stack. The mid-plane of the stack isparallel to the direction X and orthogonal to the direction Y and to theplane of FIG. 1. Included among the layers of the stack are the cover 1,the encapsulant 4 and the circuit 7. The thermally conducting cover 1has projections 2 and recesses 3 that are preferably rectangular, thatis to say have an outline that is preferably rectangular. The thermallyconducting encapsulant 4 has projections 5 and recesses 6 that arepreferably rectangular. The projections 2 and the recesses 3 of thecover 1, on the one hand, and the projections 5 and the recesses 6 ofthe encapsulant 4, on the other hand, are substantially complementary.The projections 2 of the cover 1 fit into the recesses 6 of theencapsulant 4, while the projections 5 of the encapsulant 4 fit into therecesses 3 of the cover 1. The projections 2 of the cover 1 arepreferably all identical to one another. The recesses 3 of the cover 1are preferably all identical to one another. Located on the circuit 7are components 8 connected to the circuit 7 via connection leads 9. Thelower surface 10 of the encapsulant 4, facing the circuit 7, has a shapethat is matched to the heights of the components 8 on the circuit 7 sothat, on the one hand, the stress exerted by the encapsulant 4 on thecomponents 8 and, of course, on their connection leads 9 is low enoughfor there to be no risk of damaging the components 8 and theirconnection leads 9 and so that, on the other hand, the thermalconduction between the components 8 of the circuit 7 and the encapsulant4 is high enough for there to be no risk of the components 8 heating upexcessively. The dashed arrow represents one of the many possible pathsfor draining away the energy dissipated in one of the components 8 ofthe circuit 7 first through the thermally conducting encapsulant 4toward the cover 1 and then subsequently toward the outside of theelectronic device.

[0017] So as to reduce the stress exerted by the cover 1 on theencapsulant 4 in the direction of the circuit 7, and more particularlyon the components 8 and their connection leads 9, gaps j1 and j2 areplaced along the Y axis. A first gap j1 is placed between the recesses 3of the cover 1 and the projections 5 of the encapsulant 4 and a secondgap j2 is placed between the recesses 6 of the encapsulant 4 and theprojections 2 of the cover 1. The cover 1 rests on one part of theelectronic device, which is not shown in FIG. 1 for the sake ofsimplicity. In a preferred numerical example, the gaps j1 and j2 areeach preferably around one millimeter. The depth of the projections 2 ofthe cover 1, that is to say the distance between the free end of theprojections 2 of the cover 1 and the outer surface 12 of the cover 1, ise′2 whereas the thickness of the cover 1, that is to say the dimensionof the cover 1 along the direction Y orthogonal to the mid-plane of thecover 1, is e′1. The tooling cover, shown in detail below in FIG. 3,more preferably has a thickness of e1=e′1+j1 and the depth of theseprojections will advantageously be e2=e′2+j2.

[0018] So as to maintain the thermal conduction between the encapsulant4 and the cover 1 above a given conduction threshold required by thetype of electronic device envisioned, a contact surface SC of minimumarea is provided between each projection 2 of the cover 1 on the onehand and each projection 5 of the encapsulant 4 on the other. The sum ofthe areas of all these contact surfaces SC is the overall contactsurface SG between the cover 1 and the encapsulant 4. Preferably, thegaps between projections 2 of the cover 1 on the one hand andprojections 5 of the encapsulant 4 on the other, along the X axis, areeither zero, or small enough not to disrupt the thermal conduction, oreven slightly negative in order to ensure that there is a good area ofcontact between the cover 1 and the encapsulant 4 so as to improve thethermal conduction.

[0019] The number of projections 2 of the cover 1 and projections 5 ofthe encapsulant 4, and of the recesses 3 of the cover 1 in the recesses6 of the encapsulant 4, is not critical. This number is preferably highenough to ensure an overall contact surface SG with an area sufficientto ensure the desired thermal conduction between the encapsulant 4 andthe cover 1. This number is preferably low enough to ensure, duringmanufacture of the encapsulant, preferably made of a moldable materialthat is curable or crosslinkable, a fluid flow of the material of theencapsulant by injection between the projections of a tooling cover,shown in detail below in FIG. 3, and easy demolding without any risk ofdamaging the encapsulant 4.

[0020] Since the cover 1, the encapsulant 4 and the circuit 7 constitutethe layers of a stack, the contact surfaces between recesses andprojections, facing the components 8, are preferably not parallel to themid-plane of the layers, this non-parallelism being important above allin the case of the components 8 that are particularly fragile. The fewersaid contact surfaces are parallel to the mid-plane of the layers thebetter, as the stress exerted by the cover 1 on the encapsulant 4 in thedirection of the circuit 7, and consequently on the component 8, isless. For example in the case of FIG. 1 in which the projections and therecesses are rectangular, the contact surfaces SC, on the one hand forcontact between the recesses 3 of the cover 1 and the projections 5 ofthe encapsulant 4, and, on the other hand, between the recesses 6 of theencapsulant 4 and the projections 2 of the cover 1 are not parallel tothe mid-plane of the layers of the stack, which is the planeperpendicular to the Y axis. Consequently, the only contact surfaces SCbetween the cover 1 and the encapsulant 4 are contact surfaces SCbetween the projections 2 of the cover 1 and the projections 5 of theencapsulant 4. The contact surfaces between recesses and projectionsthat are not facing components 8 or connection leads 9 are preferablynot parallel to the mid-plane of the layers, but the stress exerted onthe components 8 and on their connection leads 9 in the opposite caseentails a lower risk of damaging the components 8 or their connectionsleads 9.

[0021] Preferably, the contact surfaces between recesses andprojections, facing the components 8, are orthogonal to the mid-plane ofthe layers. The more said contact surfaces are orthogonal the better, asthe stress exerted by the cover 1 on the encapsulant 4 in the directionof the circuit 7, and consequently on the components 8, is lower. Forexample in the case of FIG. 1, in which the projections and the recessesare rectangular, the contact surfaces SC are all parallel to the Y axis.

[0022] In a plane of section orthogonal to the mid-plane of the layers,the projections advantageously have a rectangular outline. This is thecase, for example, in the electronic device shown in FIG. 1. In anotherembodiment, in a plane of section orthogonal to the mid-plane of thelayers, the projections of the cover have a trapezoidal outline, theoblique sides of the trapezoid making between them an acute angle whoseapex is directed toward the circuit 7. This type of trapezoidal outlineof the projections is also suitable but less effective than therectangular outline, as a higher stress is then exerted in the directionof the circuit 7 on the components 8 and their connection leads 9.

[0023] The sum of all of the areas of the contact surfaces SC betweenrecesses and projections amounts to an overall surface SG areapreferably greater than or equal to the area of the mid-plane of thecover 1. For example, in FIG. 1, the area of the overall surface SGwhich is the sum of all the areas of the contact surfaces between theprojections 2 of the cover 1 and the projections 5 of the encapsulant 4,is greater than the area of the interface that there would be betweenthe cover 1 and the encapsulant 4 if the surfaces facing the cover 1 andthe encapsulant 4 respectively were plane and had neither recesses norprojections. The area of the overall surface SG is then also greaterthan the area of the external surface 12 of the cover 1.

[0024] In one embodiment, the projections 2 of the cover 1 arepreferably fins; it is then easier to mount the electronic device. Inanother embodiment, the projections 2 of the cover 1 may be studs.

[0025] The cover 1 preferably also has projections on its externalsurface not facing the encapsulant 4. FIG. 2 shows schematically apreferred example of a thermally conducting cover 1 of an electronicdevice according to the invention. The thick arrows represent naturalconvection movements outside the electronic device. Projections 11 areplaced on the external surface 12 of the cover 1. The projections 11 arepreferably studs. The material of the cover 1 is thermally conductingand preferably aluminum.

[0026] The external surface 12 of the cover 1 preferably has dimensionsof at least ten centimeters in each of the directions of its plane. Thesize of the external surface 12 of the cover 1 is generally similar tothe size of the circuit 7. Advantageously, the electronic device is adigital electronic card having dimensions, for example, of around tencentimeters by around fifteen centimeters. An electronic card rackcontaining several digital electronic cards according to the inventionmay operate with less forced ventilation, or even without any forcedventilation.

[0027] The process for manufacturing a thermally conducting encapsulant4 for an electronic device according to the invention preferablyincludes a step of injecting the encapsulant 4 into a mold whichcomprises a tooling cover having, on the side intended to be in contactwith the encapsulant 4, projections substantially as wide as and deeperthan the projections 2 of the cover 1, the depth of a projection beingmeasured relative to a reference point chosen on the external surface ofthe tooling cover, so that the second gap j2 is correctly placed whenmounting the electronic device. The tooling cover preferably has agreater thickness than the cover 1 so that the first gap j1 is correctlyplaced when mounting the electronic device. The thermally conductingencapsulant preferably has a high dielectric strength. The thermallyconducting encapsulant is then preferably a moldable elastomer. Thethermally conducting encapsulant is, for example, ELASTOSIL (registeredtrade mark) RT675 from Wacker or else TSE 3281 G1 (registered trademark) from GE Silicones.

[0028] Preferably, the process for manufacturing the thermallyconducting encapsulant uses a mold that includes, facing the toolingcover, a tooling circuit similar to the circuit, and a device for atleast partially covering the tooling circuit, the covering device beinglocated between the tooling cover and the tooling circuit and beingplaced so as substantially to match the shape of the tooling circuit andso as to prevent the encapsulant from being injected beneath thecomponents or beneath the connection leads of the components of thetooling circuit. Thus, the demountable character of the electronicdevice, and more particularly of the encapsulant 4, is preserved, thusallowing maintenance and repair of the electronic device by permittingaccess to the electronic components 8 located on the circuit 7. Twodevices for at least partially covering the tooling circuit will now bedescribed with regard to FIGS. 3 and 4 respectively.

[0029]FIG. 3 shows schematically a first preferred embodiment of adevice for at least partially covering the tooling circuit used in apreferred process for manufacturing a thermally conducting encapsulantof an electronic device according to the invention. The mold includes atooling cover 21 having projections 22 and recesses 23. In the Xdirection, the projections 22 of the tooling cover 21 are advantageouslyof the same size as the projections 2 of the cover 1. In the Ydirection, the projections 22 of the tooling cover 21 have a greaterdepth than the depth of the projections 2 of the cover 1 in order tocreate the second gap j2, when mounting the electronic device accordingto the invention. The depth is measured from the external surface 26 ofthe tooling cover 21 as far as the free end of the projections 22 forthe tooling cover 21—it is equal to e2 in FIG. 3. The depth is measuredfrom the external surface 12 of the cover 1 as far as the free end ofthe projections 2 for the cover 1—it is equal to e′2 in FIG. 1. In orderfor the first gap j1 to be correctly positioned when mounting theelectronic device, the thickness of the tooling cover 21, that is to sayits dimension along the Y axis orthogonal to its mid-plane, which isequal to e1 in FIG. 3, is greater than the thickness of the cover 1which is equal to e′1 in FIG. 1. A correctly profiled tooling circuit 27or a tooling circuit 27 with components 28 and connection leads 29 has,between the tooling circuit 27 and the component 28, a space 30 that isa precluded region for demountability. Between the tooling cover 21 andthe tooling circuit 27 there is a space 24 which is the region intowhich the encapsulant 4 is injected. A frame 25 closes the mold. Thepurpose of the device 31 for at least partially covering the circuit 27is to prevent encapsulant 4 from being injected into the precludedregion 30 for demountability. If some encapsulant 4 were to be injectedinto the precluded region 30 for demountability, the encapsulant 4 couldnot be demounted and consequently the components 28 could no longer berepaired from the tooling circuit 27. The covering device 31 preferablycovers the entire tooling circuit 27. The covering device 31 ispreferably a weakly adhering film. The film 31 is, for example, made ofa polyethylene or a polypropylene or a styrenic. In FIG. 3, the film 31was shown as long bold dashed lines.

[0030] Encapsulant 4 may be injected into the injection region 24 bymeans of, for example, a hole 41 and a vent 42 in the tooling cover 21.After injection, the tooling cover 21 is withdrawn and, thanks to theweakly adhering film 31, the thermally conducting encapsulant 4 can beeasily extracted in order thereafter to be mounted in the electronicdevice.

[0031] The injection of encapsulant 4 may also be accomplished, forexample, by the encapsulant 4 flowing directly into the injection region24 under gravity in a direction perpendicular to the plane of FIG. 3.

[0032]FIG. 4 shows schematically a second preferred embodiment of adevice for at least partially covering the tooling circuit used in apreferred process for manufacturing a thermally conducting encapsulantof an electronic device according to the invention. The covering deviceis a heel 32 embedding the connection leads 29 and filling the spacebetween components 28 and tooling circuit 27, namely the precludedregion 30 for demountability. The material of the heel 32 isadvantageously an epoxy.

1. An electronic device comprising: a circuit (7) on which severalelectronic components (8) able to dissipate energy are placed; a thermalconducting cover (1) located opposite the circuit (7); a thermallyconducting encapsulant (4) placed between the circuit (7) and the cover(1) so as to ensure heat transfer, by conduction toward the cover (1),of the energy dissipated in the components (8); characterized in thatthe respective surfaces of the encapsulant (4) and of the cover (1)which are facing each other include a number of substantiallycomplementary recesses (3, 6) and projections (2, 5) allowing the cover(1) to fit into the encapsulant (4); and in that gaps (j1, j2) are leftbetween the recesses (3, 6) and the projections (2, 5) so as, on the onehand, to reduce the stress exerted by the cover (1) on the encapsulant(4) in the direction of the circuit (7) and on the other hand, tomaintain the thermal conduction between the encapsulant (4) and thecover (1) above a given conduction threshold.
 2. The electronic deviceas claimed in claim 1, characterized in that, since the cover (1), theencapsulant (4) and the circuit (7) constitute the layers of a stack,the contact surfaces (SC) between recesses (3, 6) and projections (2,5), facing the components (8), are not parallel to the mid-plane of thelayers.
 3. The electronic device as claimed in claim 2, characterized inthat the contact surfaces (SC) between recesses (3, 6) and projections(2, 5), facing the components (8) are orthogonal to the mid-plane of thelayers.
 4. The electronic device as claimed in claim 3, characterized inthat, in a plane of section orthogonal to the mid-plane of the layers,the projections (2, 5) have a rectangular outline.
 5. The electronicdevice as claimed in claim 2, characterized in that, in a plane ofsection orthogonal to the mid-plane of the layers, the projections (2)of the cover (1) have a trapezoidal outline, the oblique sides of thetrapezoid making between them an acute angle whose apex is directedtoward the circuit (7).
 6. The electronic device as claimed in any oneof claims 2 to 5, characterized in that the sum of the areas of all thecontact surfaces (SC) between recesses (3, 6) and projections (2, 5)amounts to an overall surface (SG) having an area substantially greaterthan the area of the mid-plane of the cover (1).
 7. The electronicdevice as claimed in any one of claims 1 to 6, characterized in that theprojections (2) of the cover (1) are fins.
 8. The electronic device asclaimed in any one of claims 1 to 6, characterized in that theprojections (2) of the cover (1) are studs.
 9. The electronic device asclaimed in any one of the preceding claims, characterized in that thegaps (j1, j2) between recesses (3, 6) and projections (2, 5) are of theorder of one millimeter.
 10. The electronic device as claimed in any oneof the preceding claims, characterized in that the cover (1) alsoincludes projections (11) on its external surface (12), which surface isnot facing the encapsulant (4).
 11. The electronic device as claimed inany one of the preceding claims, characterized in that the dimensions ofthe external surface (12) of the cover (1) are at least ten centimetersin each direction.
 12. The electronic device as claimed in any one ofthe preceding claims, characterized in that the cover (1) is made ofaluminum.
 13. The electronic device as claimed in any one of thepreceding claims, characterized in that the electronic device is adigital electronic card.
 14. An electronic card rack, characterized inthat it contains several digital electronic cards as claimed in claim 13and in that it is not subjected to forced ventilation.
 15. A process formanufacturing a thermally conducting encapsulant (4) for an electronicdevice as claimed in any one of claims 1 to 13, the thermally conductingencapsulant (4) having recesses (6) and projections (5) on its surfaceintended to face the cover (1), characterized in that the processincludes a step of injecting the encapsulant (4) into a mold whichcomprises a tooling cover (21) having, on the side intended to be incontact with the encapsulant (4), projections (22) substantially as wideas and deeper than the projections (2) of the cover (1), the depth (e′2,e2) of a projection (2, 22) being the distance from the free end of theprojection (2, 22) to the external surface (12, 26) of the cover (1) orof the tooling cover (21).
 16. The process for manufacturing a thermallyconducting encapsulant (4) as claimed in claim 15, characterized in thatthe thickness (e1), that is to say the dimension along the direction (Y)orthogonal to its mid-plane, of the tooling cover (21) is greater thanthe thickness (e′1) of the cover (1).
 17. The process for manufacturinga thermally conducting encapsulant (4) as claimed in either of claims 15and 16, characterized in that the mold comprises, facing the toolingcover (21), a tooling circuit (27) similar to the circuit (7), and adevice (31, 32) for at least partially covering the tooling circuit(27), the covering device (31, 32) being located between the toolingcover (21) and the tooling circuit (27) and being placed so assubstantially to match the shape of the tooling circuit (27) and so asto prevent encapsulant (4) from being injected beneath the components(28) or beneath the connection leads (29) of the components (28) of thetooling circuit (27).
 18. The process for manufacturing a thermallyconducting encapsulant (4) as claimed in claim 17, characterized in thatthe covering device (32) is a heel embedding the connection leads (29)and filling the space (30) between components (28) and tooling circuit(27).
 19. The process for manufacturing a thermally conductingencapsulant (4) as claimed in claim 18, characterized in that thematerial of the heel (32) is an epoxy.
 20. The process for manufacturinga thermally conducting encapsulant (4) as claimed in claim 17,characterized in that the covering device (31) is a weakly adheringfilm.
 21. The process for manufacturing a thermally conductingencapsulant (4) as claimed in claim 20, characterized in that the film(31) is made of a polyethylene or a polypropylene or a styrenic.
 22. Theprocess for manufacturing a thermally conducting encapsulant (4) for anelectronic device as claimed in any one of claims 15 to 21,characterized in that the thermally conducting encapsulant (4) is amoldable elastomer.